1. Field of the Invention
This invention relates to ships and excavating systems and it more particularly relates to moored offshore vessels, such as drilling and petroleum production vessels. It may equally well function as a shore based heavy duty bearing system, and as a swiveling system for stationary or dynamic loads.
2. Description of the Prior Art
Offshore vessels used in the drilling and production of petroleum, subsea mining or recovery are often best moored by a plurality of radially positioned anchors or heavy foundations set in the sea floor with mooring lines connected to the vessel. The mooring lines can be steel cables, chains, synthetic fiber ropes or a combination thereof.
Naturally, the vessel moored by this arrangement will turn in response to current, wave, and wind forces. One system allowing the vessel to weathervane employs a turret journaled in the forward part of the hull. This will allow the vessel to head into the environmental excitation vector. The mooring lines are secured to the turret, which remains stationary about a vertical axis during angular movements of the weathervaning vessel.
Priorly, the largest vessel with a turret mooring system had a deplacement of about 30,000 tons, although larger units have been proposed. The earliest turret was journaled using metal rollers on a steel raceway. Even with roller bearings, it is believed that a power assist mechanism is needed to rotate the turret to allow changing direction of the vessel. The reason for this situation resides in the coefficient of friction of metal-to-metal bearing under eccentric and dynamic load (e.g., estimated at u=0.2). Thus, on a vessel of even this modest size, the frictional forces and resulting torques resisting rotation in the turret can be of significant magnitude.
As the water depth increases wherein drilling and especially production and mining operations are desired, the size of the vessel should also increase so as to provide sufficient work room and support capability. The requirement for size naturally also increases with the magnitude of field under exploitation. In the case of a VLCC size vessel, it would measure more than 1,000 feet, and in order to take full advantage of the vessel size, a moonpool or turret diameter exceeding 70 feet is natural. A VLCC size vessel normally ranges from 200,000 to 350,000 tons displacement.
If a turret with roller bearings is to be used on a VLCC vessel, an equally distributed load in the 30,000,000 pounds range would have a frictional resistance to rotation of about 3,000 tons. It is obvious that anchor ropes cannot safely stand the resulting tortional resistance to turret rotation. Therefore, a power assisted turret rotation arrangement would have to be used to protect the anchor ropes and to protect the bearings from accelerated wear. Even if the bearing surfaces of the turret could be made from a low coefficient of friction material such as Teflon, the frictional resistance to rotation would ideally exceed 100 tons, given service loading in the 10,000-15,000 ton range. Unfortunately, the use of this bearing material would be very expensive at present day prices, and cannot conceiveably be manufactured in the necessary size, while still retaining tolerances in the ideal range.
Another problem with a turret system for any vessel is in the flexing, especially longitudinally, of the bearing surfaces. For example, the vessel in sag and hog condition changes dimension longitudinally about its neutral axis of about 0.15%. Thus, the bearing housing secured to the vessel varies in elliptical form as the vessel changes from compression to tension conditions alternately at the main deck and bottom. On a turret of the mentioned size (e.g., 70 feet), this elliptical form can be a 2.5 inch change in major and minor diameters. There are no known steel roller bearing arrangements that can accommodate this elliptical distortion condition and yet retain tolerable friction losses.
The present invention is a unique turret system with a low friction fluid bearing (like water) that can function under severe conditions of elliptical distortion while allowing a vessel subject to weathervaning almost unrestricted freedom of rotation and at loads exceeding 30,000,000 pounds on the bearing surfaces. In addition, this new turret provides a compact mounting arrangement for various anchor and riser rope handling systems, including winches, storage and utility compartments, and top mounted skid beams for the support and transportation of extremely heavy machinery.